Convection furnaces are used for a variety of applications. One particularly useful application is the reflowing of solder in the surface mounting of electronic devices to circuit boards. In such furnaces, boards, having preformed solder on the boards and devices, travel on a conveyor through a plurality of heating zones. Convection heaters above and below the conveyor heat the solder to the reflowing temperature. Fans circulate the air or other gas over the heating elements and to the boards.
Systems which employ fans have certain disadvantages however. Fans and motors which operate above ambient temperatures have reduced life spans and therefore must be continuously cooled. Also, as gas is heated, the gas expands and its density decreases. Thus, although a fan running at a constant level of power consumption moves the same volume of gas, the mass of gas moved is less. To move a mass of heated gas equivalent to a mass of cooler gas, the fan speed must be increased, with a concomitant increase in power consumption.
A further disadvantage is that during solder reflow, an effluent of vaporized flux is driven from the solder, which must be removed immediately, or it will condense on cooler surfaces, such as fan blades, with which it comes into contact. The fan blades must be regularly cleaned to prevent unbalancing and loss of efficiency.
A more recent furnace design has attempted to use a gas amplifier in the top of a sealed, pressurizable box. The gas amplifier introduces a high volume flow of air or other gas into the box. The flow circulates over heating elements to heat the gas, which pressurizes the interior of the box. The heated gas is distributed over a plate having an array of orifices and flows through the orifices to impinge on the product on the conveyor. The gas is recirculated through a return plenum.
Gas amplifiers are based upon the Coanda effect, which describes the phenomenon in which a jet of fluid exiting from a nozzle along a surface tends to follow and adhere to the surface, shown schematically in FIG. 10. If the surface is convexly curved, the jet follows the curvature of the surface. This effect is explained by the jet's entrainment of nearby ambient fluid as it flows. The entrainment of residual ambient fluid by the high velocity fluid depletes the surface fluid, so a pressure differential arises between the side of the jet adjacent the surface, where a partial vacuum or low pressure region arises, and the opposite side of the jet, which is at the ambient pressure. This pressure differential causes the jet to adhere to the surface. Continual entrainment also causes the thickness of the jet to increase. Eventually, if the surface were long enough, the jet would acquire too much mass and would break up. Henri Coanda, a Romanian scientist, studied this effect and determined the optimal curvatures for such surfaces, referred to as the Coanda profile. The profile may comprise a series of short straight segments connected to approximate a curve.
A gas amplifier comprises a body having a fluid flow passage extending therethrough from an entrance to an exit. The entrance to the passage is shaped according to a Coanda profile. A fluid under high pressure (the input) flows radially inwardly into the passage through an annular gap or space at the entrance. One surface of the gap joins and forms part of the Coanda profile of the passage through the amplifier. As the high pressure flow exits the gap, it follows the Coanda profile into the passage and thereby entrains ambient gas (the inflow) through the entrance into its flow. The output flow from the amplifier often continues to entrain some gas adjacent to the exit for a short distance. A prior art gas amplifier is shown in U.S. Pat. No. 3,806,039 to Mocarski.
The annular gap through which the pressurized gas flows typically has a width in the range of 0.001 to 0.003 inch. Small gaps produce higher output gains. However, the opposing faces which define the gap are difficult to machine accurately so that the gap width is even. Also, small gaps are subject to pluggage from small particles. Thus, spacers or shims have been used to set the gap, as shown in the patent to Mocarski. The spacer in Mocarski has radially inwardly directed teeth which set the gap width, while the input flow passes through the spaces between the teeth. The teeth, however, do not protrude into the passage, to prevent them from affecting the Coanda flow profile.
The output flow rate of the gas amplifier at the exit is the sum of the input flow rate of the pressurized fluid and the inflow flow rate of ambient fluid which becomes entrained into the input flow. The efficiency or gain, .beta., of the gas amplifier may be measured by the output flow rate divided by the input flow rate: ##EQU1## Gas amplifiers are typically used under ambient conditions to blow air for cooling or to remove particles such as dust or metal filings from an environment. In ambient environments, gains of as high as 30 or 35 have been achieved.
Gas amplifiers also have a theoretical advantage over fans and blowers in moving a heated gas. As gas is heated, for example, by flowing over heating elements, the gas expands. With a gas amplifier, the output flow increases as the gas expands, whereas with a fan, the output flow decreases, as discussed above. However, it has been observed that prior art gas amplifiers do not operate well in heated, pressurized environments, such as are found in convection furnaces. The gain is considerably less than in an ambient environment, and frequently the flow breaks down entirely. Until the present invention, the reasons for this problem apparently were not well understood.